MENDEL'S LAW AND THE HEREDITY OF ALBINISM. 393 



desires to cross his albinos with colored mice, the pedigree of the former is of con- 

 sequence. Different albinos will, in crosses with the same pigmented stock, yield 

 different results. This is shown both by Darbishire's experiments and by our own. 

 In October, 1900, we began a breeding experiment in which a family of black- white 

 mice was crossed with two different stocks of albino mice. All three stocks bred 

 true among themselves; but in crosses with the black-whites one albino stock pro- 

 duced only gray or black offspring, whereas the other produced no gray offspring, 

 but only black or fawn-colored ones, often extensively spotted with white. Mani- 

 festly the gametes formed by the two albino stocks, though all predominantly reces- 

 sive, were not all alike. It is probable that some of them at least were impure, 

 containing traces of a latent pigment-forming character. Such a latent character is 

 apparently not liberated, in the case of mice, by a cross with a different stock of albinos ; 

 but this result can be secured, probably, by a cross with dominants. We infer this 

 not only from the observed result in crosses between black-white and albino mice, 

 but also from what has been observed to take place in guinea-pigs. On crossing a 

 "dark-pointed" albino guinea-pig with a stock of red guinea-pigs which for a number 

 of generations had bred true inter se, there were obtained offspring which in every 

 instance were predominantly black in color, yet with a certain proportion of red hairs 

 mixed with the black, which gave them a "brindle" or finely mottled black-and-red 

 appearance. This result must be attributed to a liberation of the black-pigment- 

 forming character either from its visible, strict localization in the albino parent, or 

 from a possible latent and invisible occurrence in the red parent. We incline at present 

 toward the former explanation, but the matter has not yet been fully tested. The 

 complete disappearance of the albino character in this cross is noteworthy as being 

 parallel to its behavior in the cross between black-white and white mice. 



Darbishire's pink-eyed, fawn-white dancing mice were mosaics predominantly 

 recessive, and might with some propriety be designated impure recessives, but they 

 differed from impure guinea-pig recessives and from Himalayan rabbits in that, when 

 crossed with ordinary recessives, they did not produce pink-eyed animals like them- 

 selves, but rather animals which were in a majority of cases extensively pigmented. 

 It seems more appropriate, therefore, to designate them mosaics. 



"Dutch-marked" varieties of guinea-pigs, rabbits, and mice, and the somewhat 

 similarly marked Holstein and Hereford cattle, though they do not breed so true inter 

 se as dark-pointed albino guinea-pigs or Himalayan rabbits or, perhaps, as pink-eyed 

 dancing mice, nevertheless indicate a fairly precise localization of the pigment- 

 forming and albino characters within mosaic germs. 



If we adopt the Roux-Weismannian idea of the nature of the chromosomes, it 



